This invention relates in general to the field of network communication systems, to an adaptive transmission line impedance matching device and method and, more particularly, to such a device and method for xDSL and home phone line network applications.
Twisted pair telephone lines to customer premises, until recently, typically only carried voice or modulated audio in the 200 Hz to 3.4 kHz band. Therefore, optimization of the lines for transmission of signal energy primarily concerned this frequency band. A variety of techniques were developed by the telephone industry to distribute fixed value loading coils over lengths of telephone cable in order to maintain a reasonable efficiency level for the transmission line in the voice frequency band.
With the rapid evolution of digital subscriber lines (xDSL) and broadband modulation techniques in the telecommunications industry, fixed loads for high frequency impedance matching inside the broadband modem equipment have been used in place of distributed loading coil techniques. It has been generally assumed that digital signal processing algorithms used in the broadband modem equipment alone are sufficient to recover a usable amount of signal. Such is not always the case, particularly when considering lines that are many times the length of the wavelength of the frequency or frequencies being propagated.
The problem encountered is that if the signal or a portion of the signal in a given frequency spectrum is attenuated too much at the modem receiver, then it can not be recovered. High frequency receivers have a finite input dynamic range, and, once the signal falls close to or below the ambient noise level on the line, it becomes unusable. To resolve this problem, the signal reflections and attenuation due to mismatches in the transmission medium need to be significantly reduced using a well matched termination impedance or preferably a well matched transmitter impedance and termination impedance in combination with the transmission line.
Using a fixed impedance match, as is sometimes done conventionally, does increase the efficiency some. However, a problem with using a fixed impedance in the transmitter interface and/or receiver interface is that it provides only a xe2x80x9ccompromisexe2x80x9d matching impedance. Although more efficient than using no impedance matching network, there will still be a significant amount of loss of various portions of the frequency spectrum of the signal when traversing long distances over a transmission line.
Numerous problems exist in the real world that make it difficult or impossible to have even a good impedance match with a fixed load for a large range of conditions. For example, temperature variation can dynamically cause the length of several miles of environmentally exposed transmission line to change enough to dramatically adversely affect the signal being propagated.
Installing like telecommunications equipment in various countries throughout the world also makes it impossible to have a single fixed load to match all base-band impedance specifications for each country. Impedance matching devices to compensate for base-band voice or audio frequency variations from county to country, for example, are the subject of U.S. Pat. No. 5,802,169. This patent describes the implementation of a means capable of providing twisted pair line impedance matching at base-band voice or audio frequencies by selecting any of a number of predetermined value xe2x80x9ccompromisexe2x80x9d networks specific to each country. U.S. Pat. No. 5,771,262 describes an impedance matching means to account for impedance differences in various countries and covers base-band voice or audio frequencies and AMI coded digital lines inclusive of T1, E1, and ISDN.
Having unknown types of cables in a network is yet another issue. An apparatus providing a solution for such impedance matching is presented in U.S. Pat. No. 5,459,440. The system described in this patent provides real time impedance matching in order to alleviate the need for test equipment, but does give consideration to the significant loading changes found in the high frequency spectrum when base-band frequency telephone sets are randomly taken off-hook. Along with coaxial cables, the system described addresses use of twisted pair cables, but does not cover combined use of telephone and data equipment, such as local area network or the like on the same cable or twisted pair.
U.S. Pat. No. 5,459,440, like other conventional art, does not address the subject of impedance matching simultaneously for multiple frequency ranges over a broad spectrum. Performing impedance matching at high frequencies with subscriber line interface circuitry and telephone set loads being randomly attached to the twisted pair line provides a much more complex environment in which to solve the problem of automated or adaptive impedance matching.
In accordance with the present invention, an adaptive transmission line impedance matching device and method are disclosed that provide advantages over prior network communication systems.
According to the present invention, the adaptive transmission line impedance matching device and method can be used, for example, in xDSL and home phone line network applications. The adaptive impedance matching device comprises a signal transmission section, a signal receiving section, a variable impedance matching network and a signal analysis section. The signal receiving section is operable to receive signal information transmitted across a transmission line. The signal analysis section is then operable to calculate a proper impedance match based upon the signal information and to adjust the variable impedance matching network according to the calculated impedance match. The device and method are robust enough, for example, to account for dynamic mismatches when telephone sets are taken off-hook during digital (e.g., xDSL) network operation.
It is a technical advantage of the present invention to provide a unique adaptive impedance matching device to interface one or more pieces of equipment to a twisted pair telephone line, network cable, or the like. The present invention is additionally capable of separately matching a plurality of equipment segregated by operating frequency. The adaptive impedance matching device may, for example, be a single device matching the line for one or more pieces of connected equipment or the device may be separated and integrated into each piece of equipment.
It is also a technical advantage of the present invention to ensure that matching adverse impedance variations in one or more portions of the spectrum of signals transmitted on the line does not affect non-problematic portions of the spectrum.
It is another technical advantage of the present invention to continually measure the variation between the transmitted and the received signals in each portion of the frequency spectrum so as to automatically adapt the termination impedances or termination and source impedances in each respective portion of the spectrum to the attached transmission line.
It is yet another technical advantage of the present invention to use as a reference the signal or signals originated by one or more transmitters covering a wide frequency range or a plurality of segregated frequency ranges. The reference signal or signals are used to determine the characteristic impedance value as seen by the transmitter or transmitters and are additionally used to determine the appropriate load impedance value to be used by terminating circuitry in corresponding frequency bands.
A further technical advantage of the present invention is to use an adaptive means to automatically adjust the source and termination impedances of the transmission line so that optimum impedances are attained at all times. An additional benefit of automatic adjustment is that equipment users are not required to make any manual adjustments.